1. Field of the Invention
The present invention relates to a rubber composition and to a pneumatic tire using said rubber composition, and more particularly, to a rubber composition showing improved workability due to suppressing an increase with time in Mooney viscosity of a mixed rubber and improving shrinkage of the mixed rubber, and to a pneumatic tire using said rubber composition and having excellent uniformity and abrasion resistance.
2. Description of the Related Art
Recently, because of social requirements to save energy and to save resources, and particularly to cut down fuel consumption of automobiles, much research has been carried out with respect to decreasing the rolling resistance of tires and many proposals have been made.
To further improve the low heat buildup property and the low rolling resistance of a rubber composition or a tire using the same, silica is generally used in the rubber composition and a silane coupling agent such as Si69 is also used in combination.
However, a rubber composition containing silica shows very large shrinkage, causing inferior workability. Generally in a process for producing a tire, an unvulcanized rubber composition is extruded to form a prescribed shape and the extruded composition is cut to lengths prescribed for various rubber parts of the tire in accordance with the tire size. However, a rubber composition containing silica shows large shrinkage causing changes in the length after being cut and workability in tire building deteriorates to a great degree. The shrinkage is larger in portions in the vicinity of the cut surfaces of the rubber parts. As the result, the rubber parts around joint portions have larger weights and this causes a decrease in the uniformity of the tire. A rubber composition containing silica has another drawback in that the abrasion resistance thereof is inferior.
A rubber composition containing silica and Si69 shows a large increase in Mooney viscosity with time after being prepared by mixing, and workability in the extrusion step is inferior.
As for the rubber composition containing silica and a silane coupling agent, it is disclosed in International Patent Application Laid-Open No. WO97/40095 that a rubber composition comprising a conventional rubber component, silica, a specific silane coupling agent and various types of silica dispersion improvers can suppress formation of blisters during extrusion at high temperatures, prevent formation of gel, improve dispersion of silica without decreasing workability and provide a tire having excellent abrasion resistance, low heat buildup property and low rolling resistance. However, it is desired that an increase with time in Mooney viscosity of a mixed rubber composition be suppressed and shrinkage be decreased, to improve the workability and uniformity of a tire. It is disclosed in International Patent Application Laid-Open No. WO97/40096 that a rubber composition comprising a conventional rubber component and a specific silane coupling agent shows an effect similar to that shown in International Patent Application Laid-Open No. WO97/40095. However, the same drawback as that described above remains. Similar rubber compositions are disclosed in Japanese Patent Application Laid-Open (hereinafter, referred to as JP-A) Nos. 9-3245, 8-337687 and 8-333482. However, improvement in the workability and uniformity of a tire by suppressing an increase with time in Mooney viscosity of a rubber composition and by improving shrinkage is not sufficiently disclosed, not disclosed at all or not even suggested.